Conventional steering shafts include the following shock absorbing type steering shaft.
A steering shaft having a shock absorbing structure is formed of a solid round bar, and has a steering wheel connected to an upper end, and an outer sleeve connected to a lower end thereof. An energy absorber is formed between the solid bar steering shaft and outer sleeve by filling a synthetic resin into annular grooves defined on the shaft. The engagement structure consisting of the resin between the shaft and outer sleeve is broken upon collision of the vehicle. In this way, the collision energy is absorbed to prevent the steering shaft from moving backward to the chest of the driver.
The known steering shaft having the above structure defines a flat portion for torque transmission to the outer sleeve. The flat portion is formed by cutting a region of the solid round bar remote from the steering wheel. Similarly, two resin filling annular grooves, for example, are cut in the solid bar. In a region adjacent the steering wheel, the solid round bar includes an upper bearing mount, a bearing retaining nut receiving portion, a steering wheel mount, and a retainer nut receiving portion, all cut and arranged in the stated order toward an extreme end of the bar. The steering shaft is manufactured by the further steps of cutting screws in the nut receiving portions, and serrating the steering wheel mount. It is difficult for this steering shaft to be formed lightweight. Moreover, this steering shaft has the disadvantages of requiring a long cutting time, being ill suited for mass production, and involving a high manufacturing cost.